Adhesive compositions, particularly conductive adhesives, are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. The more prominent uses include bonding of electronic elements such as integrated circuit chips to lead frames or other substrates, and bonding of circuit packages or assemblies to printed wire boards. Adhesives useful for electronic packaging applications typically exhibit properties such as good mechanical strength, curing properties that do not affect the component or the carrier, and thixotropic properties compatible with application to microelectronic and semiconductor components.
When a semiconductor chip is connected to a substrate, electrical connections are made between electrical terminations on the chip and corresponding electrical terminations on the substrate. A typical method for making these connections uses metallic or polymeric material that is applied in bumps (e.g., solder bumps) to the chip or substrate terminals. The solder bumps are aligned and placed in contact and the resulting assembly heated to reflow the metallic or polymeric material and solidify the connection.
During subsequent manufacturing steps, the electronic assembly is subjected to cycles of elevated and lowered temperatures. Due to the differences in the coefficient of thermal expansion for the semiconductor chip, the interconnect material, and the substrate, this thermal cycling can stress the components of the assembly and cause it to fail. To prevent failure, the gap between the component and the substrate is filled with a polymeric material, usually referred to as an underfill, to reinforce the interconnect and to absorb some of the stress of the thermal cycling.
Two prominent uses for underfill technology are in packages known in the industry as flip-chip, in which a chip is attached to a lead frame, and ball grid array, in which a package of one or more chips is attached to a printed wire board.
The underfill encapsulation may take place after the reflow of the metallic or polymeric interconnect, or it may take place simultaneously with the reflow. If underfill encapsulation takes place after reflow of the interconnect, a measured amount of underfill encapsulant material will be dispensed along one or more peripheral sides of the electronic assembly and capillary action within the component-to-substrate gap draws the material inward. The substrate may be preheated if needed to achieve the desired level of encapsulant viscosity for the optimum capillary action. After the gap is filled, additional underfill encapsulant may be dispensed along the complete assembly periphery to help reduce stress concentrations and prolong the fatigue life of the assembled structure. The underfill encapsulant is subsequently cured to reach its optimized final properties.
If underfill encapsulation is to take place simultaneously with reflow of the solder or polymeric interconnects, the underfill encapsulant, which can include a fluxing agent if solder is the interconnect material, first is applied to either the substrate or the component; then terminals on the component and substrate are aligned and contacted and the assembly heated to reflow the metallic or polymeric interconnect material. During this heating process, curing of the underfill encapsulant occurs simultaneously with reflow of the metallic or polymeric interconnect material.
For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip and the ability to rework the failed component would be a manufacturing advantage. Thus, there is a continuing need within the semiconductor industry for underfill materials that will meet all the requirements for reinforcement of the electrical interconnect.